Power joined the Obama State Department transition team in late November 2008, and was named Special Assistant to President Obama and Senior Director for Multilateral Affairs and Human Rights on the National Security Council—responsible for running the Office of Multilateral Affairs and Human Rights—positions that she held from January 2009 to March 2013. In April 2012, Obama chose her to chair a newly formed Atrocities Prevention Board. During her time in office, Power's office focused on such issues as the reform of the UN; the promotion of women's rights and LGBT rights; the promotion of religious freedom and the protection of religious minorities; the protection of refugees; the campaign against human trafficking; and the promotion of human rights and democracy, including in the Middle East and North Africa, Sudan, and Burma. She is considered to be a key figure within the Obama administration in persuading the president to intervene militarily in Libya.[2] As of 2014, she is listed as the 63rd most powerful woman in the world by Forbes.[3]

In 2004, Power was named by Time magazine as one of the 100 most influential people in the world that year.[10] In fall 2007, she began writing a regular column for Time. Power appears in Charles Ferguson's 2007 documentary about the war in Iraq, No End in Sight, which alleges numerous missteps by the Bush administration.

Her fourth book, The Unquiet American: Richard Holbrooke in the World (edited with Derek Chollet), is an edited compilation of writings by Holbrooke's friends and colleagues, along with notable essays by Holbrooke himself.

On July 4, 2008, Power married law professor Cass Sunstein, whom she met while working on the Obama campaign.[14] They were married in the Church of Mary Immaculate, near Waterville on the Ring of Kerry in Power's native Ireland. On April 24, 2009, she gave birth to their first child, Declan Power Sunstein.[15] On June 1, 2012, she gave birth to their second child, a daughter, Rían Power Sunstein.

Alongside her advocacy for Barack Obama's candidacy, Power is best known for her efforts to increase public awareness of genocide and human rights abuses, particularly in the Darfur conflict. In 2006, she contributed to Screamers, a movie about the Darfur, Armenian, and other genocides of the 20th and 21st centuries. While Power was a leading voice calling for armed intervention to prevent mass atrocities in the Balkans and Libya,[16] she emphasizes that the United States has many non-military options for responding: "If you think of foreign policy as a toolbox, there are a whole range of options—you can convene allies, impose economic sanctions, expel ambassadors, jam hate radio. There is always something you can do."[17]

Her advocacy of humanitarian intervention has been criticized for being tendentious and militaristic, for answering a "problem from hell" with a "solution from hell."[18] Furthermore, Power's advocacy of deploying the United States armed forces to combat human rights abuses has been criticized as running contrary to the idea that the main purpose of the military is the furnishment of national defense.[19]

Some individuals have accused Power of being hostile towards Israel, largely on the basis of statements she made in a 2002 interview with Harry Kreisler. When asked what advice she would give to the president if either the Israelis or Palestinians looked "like they might be moving toward genocide," Power said that the United States might consider the deployment of a "mammoth protection force" to monitor developments between the Israelis and Palestinians, characterizing it as a regrettable but necessary "imposition of a solution on unwilling parties," and "the lesser of evils."[20] She clarified that remark on several occasions, including in an interview with Haaretz correspondent Shmuel Rosner in August 2008.[21] Many strong supporters of Israel have dismissed the charge that she is not a friend of Israel, including Alan Dershowitz,[22]Rabbi Shmuley Boteach,[23]Martin Peretz,[24] and Max Boot.[25] Boteach, who recently ran as a Republican congressional candidate in New Jersey, recalled that "she rejected utterly the notion she had any animus toward Israel [during their conversation]. She acknowledged that she had erred significantly in offering hypothetical comments that did not reflect how she felt." In an article that he published after her nomination to be U.S. ambassador to the UN, he stated that "as a Jew I am in awe of Samantha's achievement in emerging as the foremost voices against genocide in our time and I absolutely believe in her strong commitment to Israel's long-term security."[26] According to the Jewish Telegraphic Agency, "[a]fter joining the Obama team in 2009 as a member of the National Security Council — a post she left in February — she assuaged many concerns [about her commitment to Israel], first by joining [Susan] Rice in taking the lead against the singling out of Israel at the United Nations. It was Power's call, ultimately, to keep the United States out of Durban II, a 2009 reprise of the 2001 conference on racism in South Africa that devolved into a festival of Israel bashing."[27] Power's best known book, A Problem from Hell, does not discuss Israel.

Power was an early and outspoken supporter of Barack Obama. When she joined the Obama campaign as a foreign policy advisor, Men's Vogue described her as a "Harvard brainiac who can boast both a Pulitzer Prize and a mean jump shot (ask George Clooney). Now the consummate outsider is working on her inside game: D.C. politics."[28]

In August 2007 Power authored a memo titled "Conventional Washington versus the Change We Need," in which she provided one of the first comprehensive statements of Obama's approach to foreign policy. In the memo she writes: "Barack Obama's judgment is right; the conventional wisdom is wrong. We need a new era of tough, principled and engaged American diplomacy to deal with 21st century challenges."[29]

In February and March 2008, Power began an international book tour to promote her book, Chasing the Flame. Because of her involvement in the Obama campaign, many of the interviews she gave revolved around her and Barack Obama's foreign-policy views, as well as the 2008 campaign.

On February 21, Power appeared on Charlie Rose and compared Barack Obama to Sergio Vieira de Mello, who is the subject of Chasing the Flame. "This would be Sergio's lesson: if you are not thinking in terms of both dignity and freedom from fear, and this is the other thing Obama has come back to, the old Rooseveltian idea. Obama has tried to run a campaign that moves us out of the politics of fear. He is also very sensitive to the degree to which, and Sergio uses this line, 'fear is a bad adviser.' This is a line that could have come out of Obama's mouth, though happened to come out of Sergio's mouth. We make bad judgments when we are afraid."[30]

Power appeared on BBC's HARDtalk on March 6, stating that Barack Obama's pledge to "have all U.S. combat brigades out of Iraq within 16 months"[31] was a "best case scenario" that "he will revisit when he becomes president."[32] Challenged by the host as to whether this contradicted Obama's campaign commitment, she responded, "You can’t make a commitment in March 2008 about what circumstances will be like in January 2009.... He will, of course, not rely on some plan that he’s crafted as a presidential candidate or a U.S. Senator. He will rely upon a plan — an operational plan — that he pulls together in consultation with people who are on the ground to whom he doesn’t have daily access now, as a result of not being the president."[33] She concluded by saying that "what we can take seriously is that he will try to get U.S. forces out of Iraq as quickly and responsibly as possible."[32] In February 2009, Obama announced that the U.S. would end combat operations in Iraq by August 31, 2010 and withdraw all U.S. soldiers by the end of 2011. The U.S. formally ended its mission in Iraq on December 15 of that year.

In a March 6 interview with The Scotsman, she said: "We fucked up in Ohio. In Ohio, they are obsessed and Hillary is going to town on it, because she knows Ohio's the only place they can win".[34][35] "She is a monster, too — that is off the record — she is stooping to anything... if you are poor and she is telling you some story about how Obama is going to take your job away, maybe it will be more effective. The amount of deceit she has put forward is really unattractive."

Power apologized for the remarks on the night of the March 6 interview, saying that they "do not reflect my feelings about Sen. Clinton, whose leadership and public service I have long admired," and telling Irish TV reporter Michael Fisher: "Of course I regret them. I can't even believe they came out of my mouth....in every public appearance I've ever made talking about Senator Clinton, I have sung her praises as the leader she has been, the intellect. She's also incredibly warm, funny....I wish I could go back in time."[36] The next day, in the wake of reaction to the remarks, she resigned from the Obama campaign.[37] Soon afterward, the Weekly Standard said that it "might have been the most ill-starred book tour since the invention of movable type."[38]

Following her resignation, she also appeared on The Colbert Report on March 17, 2008, saying, "can I just clarify and say, I don't think Hillary Clinton is a monster...we have three amazing candidates left in the race." When Power later joined the State Department transition team, an official close to the transition said Power had apologized and that her "gesture to bury the hatchet" with Clinton had been well received.[39] Power attended Clinton's swearing-in ceremony on February 2 and collaborated with her during her four-year tenure as Secretary of State.

Samantha Power is considered to be a key figure within the Obama administration in persuading the president to intervene militarily in Libya.[43] Power argues that America has a moral obligation to examine all tools in the toolbox (diplomatic, economic, political, and military) to respond to mass atrocity, and she has argued that there may be circumstances in which military intervention may be appropriate to prevent genocides.[citation needed] Within the White House, Power strongly pressed for U.S. intervention on humanitarian grounds.[citation needed] She has been described as instrumental in convincing Obama to push for a UN Security Council resolution to authorize a coalition military force to protect Libyan civilians.[44] Power has previously argued that "you don't get any extra credit for doing the right thing". "It's up to us" to change that calculus, she said. "My prescription," she said, "would be that the level of American and international engagement would ratchet up commensurate with the abuse on the ground."[45]

On June 5, 2013, the President Barack Obama announced her nomination as the new United States Ambassador to the United Nations.[46] In remarks at the announcement, Power said: "Even as a little girl with a thick Dublin accent who had never been to America, I knew that the American flag was the symbol of fortune and of freedom....I came home from school every day, as my mother can attest, my dad can attest, and I sat in front of the mirrors for hours, straining to drop my brogue so that I, too, could quickly speak and be American."[47]

Senator John McCain (R-AZ) said: "I support President Obama's nomination of Samantha Power to become the next U.S. Ambassador to the United Nations. I believe she is well-qualified for this important position and hope the Senate will move forward on her nomination as soon as possible."[48]Senator Lindsey Graham (R-SC) called her "solid choice to serve as United States Ambassador to the United Nations. As United Nations Ambassador, Samantha Power will aggressively represent the United States interests in an increasingly hostile body. Power will also be a strong supporter of the United States' close ally Israel.[49] Former Senator Joseph Lieberman (I-CT) said that he is "very encouraged by the president's appointment."[50]Dennis Ross stated that "Sam Power brings lots of experience to the job and will be a powerful voice representing America's interests and values."[51]Abraham Foxman, national director of the Anti-Defamation League, wrote: "We are pleased that President Obama has nominated a true champion of human rights who led the effort to make averting genocide and atrocities a core part of American policy to head the U.S. delegation at the United Nations...She experienced first-hand the hostility faced by Israel and the abuse of the U.N. bodies to promote anti-Israel bias. As someone who appreciates, to the core of her being, the meaning of international human rights mechanisms, Samantha is clear eyed and understands the injustice of their abuse to target Israel's legitimacy."[52] Israel's Ambassador to the U.S., Michael Oren, noted that "Samantha Power and I have worked closely over the last four years on issues vital to Israel's security. She thoroughly understands those issues and cares deeply about them."[53] Professor Dershowitz reiterated his strong support for Power: "She's a perfect choice. A perfect choice. She has real credibility to expose the U.N.'s double standard on human rights. She also understands the principle of 'the worst first'—you go after the worst human rights abusers first."[22] Lawyers for Cholera Victims also see her nomination as an opportunity for the US to pressure the UN to respond justly to the cholera epidemic. Director of the Institute for Justice & Democracy in Haiti cites her "long and effective record of support for the rule of law, especially international law,” as a sign that she will stand up for accountability for the victims of UN cholera in Haiti.[54] The Director of the Israel Project, Josh Block, noted that "Samantha has made a commendable effort to build ties with the pro-Israel community and develop deeper appreciation of the issues vital to our interests in the region, Israel's security, and the U.S.-Israel relationship.

In her role at the United States National Security Council, she also helped lead the administration's efforts opposing the Palestinian bid to circumvent peace negotiations with Israel with unilateralism at the U.N."[55] The Jewish Council for Public Affairs noted that it has "worked closely with Samantha Power over the years in her roles as journalist, activist, and government official. Power has been a critical voice on human rights issues and we are very proud of our joint work to confront atrocities, including that in Darfur and the ongoing crisis in Sudan."[56] The President of the Rabbinical Assembly, Gerald Skolnik, said that the Assembly "look[s] forward to working with Samantha Power in her new role as UN Ambassador on our mutual interests of defending universal human rights and Israeli security.[57] The Eastern Director of the Simon Wiesenthal Center, Steven Burg, tweeted that she is a "great choice."[58] The National Jewish Democratic Council said that "Power, the country's foremost scholar on genocide and an outspoken advocate for human rights, represents some of the Jewish community's most important values."[59] A recently posted document gives several examples of pro-Israel, Republican support for her.[60]

Her nomination was also opposed by a number of people. Former US ambassador to the UN John R. Bolton and former US Assistant Secretary of Defense for International Security Policy Frank Gaffney criticized her for a 2003 article in The New Republic that she wrote in which she compared the United States to Nazi Germany.[61]

Speaking in September 2013, Power told a news conference that the American intelligence findings “overwhelmingly point to one stark conclusion: The Assad regime perpetrated an attack.” She added, “The actions of the Assad regime are morally reprehensible, and they violate clearly established international norms.”

Power went on to criticize the failure of the United Nations structure to thwart or prosecute the atrocities committed in the Syrian conflict, which is now well into its third year. She said, “The system devised in 1945 precisely to deal with threats of this nature did not work as it was supposed to.”

She added "Even in the wake of the flagrant shattering of the international norm against chemical weapons use, Russia continues to hold the council hostage and shirk its international responsibilities. "What we have learned, what the Syrian people have learned, is that the Security Council the world needs to deal with this crisis is not the Security Council we have."[64]

In 2014, speaking on the crisis in Ukraine, Ambassador Power, told reporters that Washington was "gravely disturbed by reports of Russian military deployments into the Crimea. "The United States calls upon Russia to pull back the military forces that are being built up in the region, to stand down, and to allow the Ukrainian people the opportunity to pursue their own government, create their own destiny and to do so freely without intimidation or fear," she said. Power declined to characterise Russian military actions when asked if they constituted aggression. She called for an independent international mediation mission to be quickly dispatched to Ukraine.[65]

In July 2014 Power said the GLBT rights movement is “far from over” in spite of significant progress in this country. “There are some parts of the world where the situation abroad is actually taking a sharp turn for the worse for LGBT individuals,” she said during a forum at Hunter College commemorating the 45th anniversary of the Stonewall riots. Power noted that homosexuality remains criminalized in nearly 80 countries. She said Brunei would become the eighth country in which those found guilty of consensual same-sex sexual acts face the death penalty if it “continues along its path” of enacting its new penal code. Power highlighted the law that Ugandan President Yoweri Museveni signed in February 2014 that imposes a life sentence upon anyone found guilty of repeated same-sex sexual acts. She also noted Russia and Nigeria as among the other countries in which anti-LGBT statutes have come into effect over the last year. “Unfortunately, Uganda’s anti-gay legislation is not an outlier,” said Power. “Nor is the climate of intolerance and abuse that it has fostered.” Power spoke at the forum a week after the Obama administration announced travel bans against Ugandan officials who are responsible for anti-LGBT human rights abuses. Her speech also coincided with the first anniversary of the landmark U.S. Supreme Court decision that struck down a portion of the Defense of Marriage Act.[66]

As of 2014, she is listed as the 63rd most powerful woman in the world by Forbes.[3]